The present invention relates to a process for preparing a solid component of a high-yield catalyst for the polymerisation of xcex1-olefins.
High yield heterogeneous supported catalysts for olefin polymerisation can be obtained, for example, by contacting:
a solid catalytic component obtained by contacting a titanium compound comprising at least a titanium-halogen bond with a solid support comprising a magnesium halide or a compound capable of reacting with the above said titanium compound to yield a magnesium halide;
an aluminium organometallic compound; and optionally
an electron donor compound (external donor).
The above mentioned reaction can be carried out in the presence of another electron donor compound (internal donor). This is necessary when a catalyst is produced for the stereospecific polymerisation of propylene or higher xcex1-olefins.
Useful solid catalytic components are obtained by reacting TiCl4 with a support consisting of a magnesium compound that can be a magnesium dihalide, such as MgCl2, or an alcoholate or haloalcoholates of magnesium, such as ethoxymagnesiumchloride or diethoxymagnesium. Particularly preferred supports consist of adducts of MgCl2 with aliphatic alcohols, such as ethanol, in the form of spherical particles.
It is known that the preparation of such solid catalytic components is carried out in a batch reactor. The use of a batch reactor gives problems due to the formation of by-products deriving from the reaction of the titanium compound with the magnesium compound and with the electron donor compound, if present. This fact limits the solids concentration (with respect to the liquid titanium compound) that can be used in the contacting step and requires many washing steps of the obtained component. Consequently, the preparation of solid catalytic components in a batch reactor requires long residence times, large reactor volumes and large amounts of liquid titanium compound, thus raising the production costs. Furthermore, the presence of sensible concentrations of the said by-products affects the catalyst activity and its stereospecificity, when propylene or higher xcex1-olefins are polymerised.
With the aim of simplifying the preparation of such catalyst components, Italian patent No. 1,188,169 suggests percolating a liquid comprising a halogenated titanium compound through a solid support consisting of a magnesium halide in active form. However, inasmuch as the percolation consists of the gravity flow of a liquid through the pore spaces and interstices of solid particles, the composition of the percolating liquid varies while flowing through the solid and the concentration of the system is far from being uniform. Moreover, the percolation process described in this Italian patent does not allow obtaining a catalyst endowed with a satisfactory activity.
It has now been found that by carrying out the preparation of the above catalyst component in a continues way and by selecting properly the operative conditions in the step of titanation, the drawbacks due to the presence of by-products are minimised and the productivity of the process itself is increased. Furthermore, the final catalysts are endowed with good performances as to activity and stereospecificity.
It is therefore an object of the present invention a process for preparing a solid catalyst component for the polymerisation of olefins, said process comprising the steps of:
continuously feeding a liquid containing a titanium compound having at least one titanium-halogen bond into a vessel containing a suspension of a solid comprising a magnesium halide, and
continuously discharging liquid from the vessel,
wherein the suspension is maintained under adequate stirring so as to obtain substantial uniformity of concentration throughout the vessel.
In the process of the present invention, the concentration of the solid is maintained within the range of between 80 and 300 g/l, preferably between 100 and 250 g/l, and the product between the average residence time of the liquid in the vessel and the concentration of the solid is maintained below 10,000 min*g/l, preferably below 5,000 min*g/l.
In order to maintain said product at a value below 10,000 min*g/l, the average residence time of the liquid in the vessel is preferably below 60 min, more preferably below 50 min, and it can be advantageously kept even under 30 min.
As oppose to the small mixing of the liquid that flows under the gravity action in a plug flow mode in the percolation, the technique adopted in the process of the present invention makes it possible to achieve an intimate mixing of the contents of the reactor. The resulting uniformity of concentration, and also of temperature, throughout the reactor accounts for the better effectiveness and efficiency of the process. The present reactor approaches the ideal mixing pattern described by G. F. Froment and K. B. Bishoff in Chapter 10 of xe2x80x9cChemical Reactor Analysis and Designxe2x80x9d John Whiley and Sons (1979).
To continuously withdraw liquid from the vessel, the liquid needs to be separated from the solids in the slurry. This can be achieved by means of appropriate separating techniques, particularly by filtration or centrifugation. Suitable separating elements are filters that are generally placed inside the vessel but can also be placed outside of the vessel. Alternatively, a centrifuge or a hydrocyclone can be employed.
An electron donor compound (internal donor) may be added to the support before or during the contacting with the titanium compound. It can also be added along with the liquid containing the said titanium compound.
A preferred titanium compound is TiCl4.
The solid comprising the magnesium halide may be advantageously suspended in a liquid which can be a titanium compound, an organic compound or a mixture thereof.
Alternatively, a magnesium compound capable to yield a solid magnesium halide by reaction with the said titanium compound can be introduced into the vessel and reacted with the said titanium compound.
Preferred magnesium compounds are magnesium halides, such as MgCl2, or compounds capable to yield MgCl2 by reaction with the titanium compound. Particularly preferred magnesium compounds are magnesium adducts with an aliphatic alcohol, preferably ethanol. It is preferred that the magnesium halide is in its active form and that the magnesium compound is capable to yield a magnesium halide in its active form when reacted with the Ti compound. As it is well known in the field of Ziegler-Natta catalysts, xe2x80x9cmagnesium chloride in active formxe2x80x9d means magnesium chloride characterised by X-ray spectra in which the most intense diffraction line that appears in the spectrum of the non-active halide is diminished in intensity and is replaced by a halo whose maximum intensity is displaced towards lower angles relative to that of the more intense line. Particularly preferred magnesium compounds are spheriform adducts MgCl2/ethanol with average diameter in the range of between 0.1 and 150 xcexcm, more preferably of between 1 and 100 xcexcm.
Electron donor compounds suitable for the preparation of the solid catalyst component can be selected from ethers, esters, ketones, amides and tertiary amines. One class of preferred electron donors is the mono- or di-alkyl esters of aromatic carboxylic acids, such as diisobutylphthalate or ethylbenzoate.
It is preferred that the liquid fed to the vessel during the contacting treatment contains, at least from a certain time on, a liquid organic substance having dielectric constant at 20xc2x0 C. equal to or higher then 2 such as those described in EP-106,141. Preferred liquid organic substances are aromatic hydrocarbons or aromatic halohydrocarbons. The use of aromatic halohydrocarbons, such as chlorinated aromatic hydrocarbons, may lead to superior activities. In the class of non-halogenated hydrocarbons, toluene and ethylbenzene are particularly preferred. A suitable composition for the liquid phase fed to the vessel is an equimolecular mixture of the titanium compound and of an aromatic hydrocarbon.
The initial contacting temperature of the liquid comprising a titanium compound with the solid support is generally in the range of between 0 and 50xc2x0 C. Such a temperature is then gradually raised to the value to be kept in the range from 40xc2x0 C. to 200xc2x0 C., preferably from 80xc2x0 C. to 135xc2x0 C.
The feeding of the electron donor compound is preferably carried out when the temperature of the vessel ranges from 0xc2x0 C. to 80xc2x0 C., more preferably from 20xc2x0 C. to 60xc2x0 C.
As an example, a typical scheme for preparing a catalyst component for the stereospecific polymerisation of propylene according to the process of the invention is given below.
a vessel is loaded with TiCl4 at a temperature of 0xc2x0 C.;
then the solid support (MgC2*nEtOH, where n is from 0.1 to 6) is loaded;
the temperature is then gradually raised to 40xc2x0 C.;
next the electron donor (for example diisobutylphthalate) is fed;
thereafter TiCl4 is continuously fed and the liquid phase continuously withdrawn to keep the solids concentration at 200 g/l, the average residence time of the liquid being 15 minutes, while the temperature is gradually raised to 120xc2x0 C.;
when the temperature of 120xc2x0 C. is reached, the composition of the liquid being continuously fed is changed to a mixture of a liquid aromatic hydrocarbon and TiCl4 in equal parts by volume, the average residence time of the liquid being 15 minutes and the solids concentration and the temperature being kept constant for 1 hour;
after that time the liquid feeding is discontinued and the liquid phase is removed;
finally the solid is washed with toluene at 100xc2x0 C. and then 5 times with hexane at 60xc2x0 C.
An alternative way to proceed is to load the vessel with TiCl4 at 10xc2x0 C., then introduce the solid support and the electron donor, keep for 1 hour at 10xc2x0 C. and then the continuous feeding of the TiCl4 is started and the process is conducted as above.
According to a preferred embodiment, the process can be carried out as follows:
a vessel is loaded with an organic solvent (for example heptane or toluene) at 10xc2x0 C.;
then the solid support (MgCl2*nEtOH, where n is from 0.1 to 6) is loaded.
thereafter TiCl4 is continuously fed and the liquid phase continuously withdrawn to keep the solids concentration between 100 and 200 g/l, the average residence time of the liquid being below 50 minutes, while the temperature is kept at 10xc2x0 C.;
subsequently the electron donor is introduced in about 10-30 minutes;
after 1 hour the temperature is raised to about 110xc2x0 C. and kept for another hour, while the TiCl4 continues to be fed;
after that time the liquid feeding is discontinued and the liquid phase is removed;
finally the solid is washed with toluene at 100xc2x0 C. and then 5 times with hexane at 60xc2x0 C.
It has been found that catalysts obtained with the latter procedure are generally capable to yield polymers endowed with greater porosity.
The catalyst components produced by the process of the present invention are generally used after contacting them with an aluminium compound such as an aluminium-trialkyl or an aluminium-alkyl-hydride. A commonly used compound is triethyl-aluminium.
Particularly when stereospecific polymers are produced, an electron donor (external donor) is contacted with the solid catalyst component before the polymerisation. The external donor compound may be the same or different from the internal donor. Preferred compounds are silicon compounds containing at least one Sixe2x80x94OR bond (R being a hydrocarbon radical).
The catalysts obtainable from the solid catalyst components prepared with the process of the present invention are particularly suitable to homo- or co-polymerise xcex1-olefins of formula CH2xe2x80x94CHR, wherein R is hydrogen or an alkyl, cycloalkyl, aryl, arylalkyl or alkylaryl radical with 1 to 12 carbon atoms. A preferred field of application is the production of stereospecific (co)polymers of propylene.
The following examples will further illustrate the present invention without limiting its scope.
Characterisations
Isotactic index: insolubility in xylene at 25xc2x0 C.
Bulk density: DIN-53794.
Porosity: determined by immersing a known quantity of the sample in a known quantity of mercury inside a dilatometer and then gradually increasing the mercury pressure hydraulically. The pressure of introduction of the mercury into the pores is function of the pore diameter. Measurements were effected using a xe2x80x9cPorosimeter 2000 seriesxe2x80x9d from Carlo Erba. The porosity, pore distribution and surface area were calculated from the data of decrease of the volume of mercury and from the values of the applied pressure.
Preparation of the Solid Support
A solid support consisting of a MgCl2*2.1C2H5OH adduct was prepared following the procedure described in Example 1 of EP A 728 769, from page 12, line 55 to page 13, line 13.
Preparation of the Solid Catalytic Component